Organic Light Emitting Displays (OLED) can be divided into passive matrix and active matrix according to driving methods. The so-called active matrix OLED (AMOLED) is to use a thin film transistor (TFT) and the capacitor to store signals and control the luminance and the gray scale of OLED.
For driving technology at present, development of AMOLED has two directions; one is the analog method and the other is the digital way. The reason why digital driving was developed is because TFT elements with uniform features (e.g. threshold voltage and mobility) can't be produced through the current LTPS process. Nevertheless, the stringent demands for LTPS process are not required for digital driving since characteristic variation of TFT elements can be compensated merely through a simple 2T1C driving circuit.
As a result, digital driving technology will play a certain role for the development of AMOLED in the future if shortcomings of digital driving method can be corrected efficiently and the integrated driving system can be established.
The driving structure of the digital driving technology in practice is based on Program Display Separation as shown in FIG. 1. One defect of this method is the low time utility rate since OLED is not allowed to be illuminated during sub-frame writing time from sub-frame SF1 through SF6 and the total writing time from sub-frame SF1 through SF6 occupies a certain portion of frame time. 1˜N refers to the scan line and 1˜M refers to the display line. For each sub-frame (SF1˜SF6), the writing time is the same and the luminance time is T, 2T, 4T, 8T, 16T and 32T in order respectively. Take FIG. 1 as an example. When the resolution of the display panel is 176×240 with the scanning frequency of 120 KHz, the writing time length of a sub-frame equals to (1/120K)×240=2 ms. Consequently, the total writing time for 6 sub-frames SF1˜SF6 will be 12 ms, which occupies 60% of a frame time (1 frame= 1/50 sec=20 ms). As OLED is not illuminated during writing time, the time utility rate only achieves 40%, which is low and might lead to insufficient brightness of the display panel.
Take the U.S. Pat. No. 6,452,341 as an example for time-ratio technology. It is based on the structure of Program Display Separation for the realization of digital driving. Though this approach is easy to implement and the hardware system is less complicated; however, time utility rate is low since the total writing time from sub-frame SF1 through SF6 occupies a certain portion of frame time.
Japan Pat. No. 2001-343933 discloses a method for driving AMOLED. The driving elements in every pixel include a writing TFT, an erase TFT, a driving TFT, a storage capacitance, and an organic electro-luminescence element. The gate of the writing TFT is connected to the write scan line and the gate of the erase TFT is connected to the erase scan line. Gray scale is adjusted by modulating the luminance time ratio in this patent, which improves the flaw of low time utility rate in the driving structure of program display separation. Whereas, driving elements with three TFT (3T1C) are required leading an improvement in complexity of the driving method and aperture ratio of pixels to be desired.